Users of industrial size shipping containers such as 55 gallon steel drums now demand a very high standard of drum interior purity due to the super sensitive nature of many drummed products. This requirement has placed a burden on the drum manufacturer to assure delivery of drums absolutely free of interior contamination. One step in the drum making process where such contamination was heretofore likely to occur involved the exterior spray painting of the completed drum. Specifically, the problem has been one of adequately covering the threaded dispensing openings in the drum head to prevent the entry of spray paint. Such openings are commonly surrounded by an upstanding neck terminating in an outwardly curled bead. Besides preventing paint entry, it is also necessary to protect the upwardly exposed bead surface from paint coverage so that subsequent plugging operations do not chip any paint particles into the drum. Consequently, it has become common practice to apply some type of protective cover over the drum openings prior to the paint operation.
In addition to meeting the above essential criteria, it is of equal importance that any protective closure employed by quickly and easily applied and removed without, in itself, inflicting any damage or detrimental abrasive action on the threaded opening. Another consideration in the drum painting process which is carried out by rotating the drum about its longitudinal axis in the path of a fixed paint gun, is the degree of undesirable shielding caused by a protective closure. What may seem like a very minor change in the opening neck profile due to the presence of a protective closure, can seriously detract from the amount of paint coverage obtainable on the opening neck exterior immediately below the bead. This condition is objectionable as it leaves a band of unpainted metal round the opening which quickly rusts. Accordingly, a balance must be struck with the protective closure between keeping undesirable paint off of the critical surfaces of the opening bead and thread on the one hand while enabling complete coverage of the lower exterior neck surfaces on the other hand.
Yet another consideration comes into being when the painted drum is then conveyed through the curing oven. The elevated baking temperature causes air inside the drum to rapidly expand which unless vented to the outside, will bulge and damage the drum. Once again, a satisfactory protective closure must prevent the entry of paint at one step in the process and let expanding hot air escape at another step. Moreover, the closure must resist dislodgement or blow off at one point and be easily and quickly removed when hot exiting the oven at another point. The task is formidable.
Of the many attempts at solving the problem a few bear mentioning. A common approach has been to simply thread an ungasketed closure plug into the opening neck. This leaves the entire bead surface surrounding the opening exposed to paint coverage and is unsatisfactory. Another approach has been to apply an overcap which has a series of spring fingers that releasabily snap over the opening bead. This has a shielding effect causing incomplete paint coverage of the lower neck surfaces. Yet another approach has been to employ a protective cap having a crown-like fluted skirt which engages over the maximum outside diameter of the opening bead. This cannot accommodate the range of bead tolerances from drum to drum and gets blown off when going through the curing oven. Still another approach has been to employ a large grippable disc having a pair of radially outwardly spring biased legs depending from the undersurface which are compressed radially inwardly when forced into the openings so as to hold the disc on the bead. This construction is far too abrasive on the opening threads is too costly and shields the exterior of the neck too much. A simple solution to this complex problem has been elusive.